In recent years, a display device provided with a light-detecting sensor (also referred to as a photosensor) has attracted attention. In the display device including a photosensor, a display screen also serves as an input region. A display device having an image pickup function is an example of such a display device (see Patent Document 1, for example).
Examples of a semiconductor device provided with a photosensor are a CCD image sensor and a CMOS image sensor. Such image sensors are used in, for example, electronic apparatuses like digital still cameras or cellular phones.
In a display device including a photosensor, first, light is emitted from the display device. When the light enters a region where an object to be detected exists, the light is blocked by the object to be detected, and is partly reflected. The light reflected by the object to be detected is detected by the photosensor provided in a pixel in the display device, whereby the object to be detected can be found in the region.
In a semiconductor device including a photosensor, light emitted from an object to be detected or external light reflected by the object to be detected is detected directly by the photosensor or condensed by an optical lens or the like and then detected.